Protector



Patented Aug. 8, 1939 PROTECTOR-. A

Earl L. Fisher, Newark, N. J.,

Telephone Laboratories,

asnsignor to Bell Incorporated, New

York, N. Y., a corporation of New York Applicationugust 31, 1937, SerialNo. 161,753

a claims. This invention relates tb protectors and more specifically toprotectors for telephone substation equipment.

Protectors of this type usually comprise a high resistance path fromeach line conductor to ground. These paths are of such high resistanceas to prevent for practical purposes the flow of the low voltagetelephone and ringing currents to ground. However, if a high voltagefrom a power line, lightning or other source is accidentally applied tothe line it passes over the'high resistance path to ground rather thanthrough the equipment. A common form of protector is an air-gap betweenline and ground, -which air-gap is broken down by the application of ahigh voltage, allowing current to ow to ground. A more recentdevelopment employs in place of the airgap, a resistance material havinga non-linear voltage-current characteristic and commonly referred to asa non-ohmic material. Since with such a material the resistancedecreases with increase of applied potential, this material may bedefined as one having an inverse resistance-voltage characteristic.Materials which have been found to' possess the desired characteristicsare 'Ihyrite, carborundum and certain other refractory materials. Thepreferred material for this protector is Thyrite, which has been knownin the electrical art for some time. Its physical properties and some ofits applications are described by T. Brownlee, General Electric Review,vol. 37, No. 4, April 1934, pages 175 et seq. The advantage of this typeof protector is that it is capable of withstanding repeated dischargesdue to abnormal potentials.

The inherently higher resistance of this type of protector, however, mayproduce a re hazard under conditions of long contact with the source ofhigh voltage, such as an adjacent power circuit, unless units ofimpracticable size are used or unless some special preventive auxiliarydevice is employed.

An object of this invention is to combine a protector of this type witha thermally responsive means to short-circuit the protector to groundbefore a dangerously high temperature exists.

A further object is to employ a mounting for the protector in which thegeneration of excessive heat by the protector will cause a thermostaticmetal member to short-circuit the protector.

A feature of the invention is the production of a substationprotectorunit which includes a conductor element having an inverseresistance-voltage characteristic as the high resistance path and a.mounting in which is incorporated a thermally responsive means forshort-circulting the high resistance only during the persistence of anabnormally high temperature in the unit.

An exemplary embodiment of the invention is illustrated in theaccompanying drawing in 5 which:

Fig. l is a perspective view of the protector device mounted on asuitable base;

Fig. 2 is a central section taken at right angles to the plane of theprotector disc; and 10.

Fig. 3 is a partial section taken on a plane parallel to the protectordisc.

The protector is mounted on a perforated base member I of ceramic orother suitable material. The ground contact element 2 is in the form ofl5 a disc with a reduced portion 3, which has a right angle projection 4which comprises a combined support and connecting means. The parti ismounted on the base I by means of the terminal bolt 5 and nut 6.Projected from either side of 20 the portion 3 and rmly attached theretoare ange members 1. These members provide additional support for theground contact element 2 and also carry the auxiliary ground contactmember 8, the function of which will be described 25 in due course.

On either side of the ground contact 2 are the line contacts I0. Theseare in the form of discs of the same size as the ground contact discs.

These contacts are supported on the base by 30 resilient bracket membersIl which also serve as electrical connectors between the contacts IIIand the terminals I2. The terminals I2 are in the form of bolts by meansof which the brackets are attached to the base. The contact discs I0 35are slotted part Way through the vertical diameter as at I 3. Within thevertical slots are located thermally warpable strips I4. 'I'he contactsI0, brackets II and strips I4 of each group are secured together by thefastening means I5 which 40 may be screws or rivets. Between the groundcontact 2 and each line contact l0 is mounted a disc I6 of Thyrite orother material having the desired resistance-Voltage characteristic.'Ihese discs I6 are frictionally held in place due to the 45 pressureapplied by the resilient bracket members Il.

In practice, connection is made from terminal 5 to ground and from eachof terminals I2 to a line conductor. 50

The operation of the device is as follows:

Under conditions of normal Voltage the very high resistance of theconductors I6 prevents for all practical purposes, the iiow of currentfrom the line wire to ground. However, upon the 55 application oiabnormally high potentials the resistance of members I6 goes down andcurrent ows to ground. If this Aflow is sustained for a predeterminedperiod suiiicient heat may be generated to cause the thermallyresponsive members I4 to deflect. These members are arranged to warpoutwardly from the slot I3 into contact with the auxiliary groundcontacts 8. This provides a low resistance shunt around the disc i6. Thegeneration of heat to a dangerous degree is thus prevented. Uponreduction of temperature the thermal elements Il recede, opening theshunt path. If the high voltage persists the cycle is repeated. However,if the high voltage has been removed the protector assumes normaleffective condition. By means oi this invention continuous protection isachieved Without the disadvantages attendant upon use of otherprotectors of this general type.

Although the invention has been described with relation to theparticular embodiment illustrated in the drawing, it is understood thatit is not limited thereby but only by the scope of t the appendedclaims.

auxiliary contact connected to the other electrode and located adjacentsaid slot in a position to be contacted by the bimetalllc element when adangerously high temperature exists in the device.

2. A protective device comprising a pair of electrodes separated by amember of 'high resistance material having an inverse resistancevoltagecharacteristic, a bimetallic elementmounted in a slot in one of saidelectrodes and having one end in contact with said high resistancematerial, and an auxiliary contact connected to the other electrode andlocated adjacent said slot in a position to be contacted by thebimetallic element upon a predetermined increase in temperature of saiddevice.

3. A protective device comprising a pair of electrodes separated by amember of high resistance material having an inverse resistancevoltagecharacteristic, a bimetaliic element mounted in and substantiallyclosing a slot in one of said electrodes and vhaving one end in contactwith said high resistance material, and an auxiliary contact connectedto the other electrode and located adjacent said slot in a position tobe contacted by the bimetallic element upon a predetermined increase intemperature of said device.

EARL L. FISI-IER.

